Magnesium oxide coating method



United States Patent 3,181,964 MAGNESIUM OXIDE COATING METHOD Bruce E. Kramer, Loveland, and Eugene J. Fields, Cincinnati, Ohio, assignors to the United States of America as represented by the Secretary of the Navy 3, No Drawing. Filed Apr. 25, 1962, Ser. No. 190,185

6 Claims. (Cl. 117-23) it This invention is directed toward forming a filn'l of metal oxide and, more particularly, to a method of hotspraying magnesium oxide coatings to solid objects.

Metal oxide coatings demonstrate excellent resistant to heat and abrasion. and moreover, they embody exce'flent thermal shock characteristics. Metal oxides melting above 1000 C. are ideally suited as coatings for construction material, the surfaces of which are subjected to conditions of high temperatures, severe oxidation and to either extreme erosiveness or abrasiveness.

Although metal oxide coatings have been known in the art for many years, the advent of supersonic aircraft, jet engines and rocket propelled missiles has increased greatly the possible field of utility for these coatings with their desirable characteristics.

Under the prior art, it is the practice to apply tlti ese coatings to solid objects by metal spraying. Flame sprayed coatings of the prior art have rangedfrom pure or sibstantially pure metallic coatings, on the one hand, to coatings consisting essentially of high melting metal oxides.

Magnesium oxide has long been known to be especially desirable as an oxide coating because it is exceedingly refractory and insulative and highly serviceable for this purpose. 7

In general; heretofore, the flame sprayed coatings are made by melting and atomizing the metal oxide or a mixture of a metal and its metal oxide (seeding effect).

by means of heat from a flame of a burning fuel such as an oxyacetylene or an oxy-hydrogcn flame. As a rule, metal spraying is carried out with the use of metal spray guns, i.e., devices in which the metal is fed to a heating zone from which zone metal particles, at least some of which are molten or in a heat plastic condition, are propelled against the'surface to be sprayed. This propulsion may be effected by a blast of air or other gas.

One of the most widely used type of spraying device? is a metal spray gun which utilizes the metal to be sprayed in the form of a rod or wire which is fed by a suitable feed mechanism into a heating zone which is produced by the combustion of a combustible and a combustion supporting gas.

Previous attempts to flame spray magnesium oxide in rod or powder form were unsuccessful because its melting point and boiling point are only 20 F. apart. The vaporization of the magnesium resulted in the magnesium coatings deposited by this method being soft and porous. In addition to which the magnesium coatings did'not adhere very strongly to the substrate.

There is, therefore, a need for an improved method of forming a continuous adhesive coating or film of magnesium oxide on a metal base and for magnesium oxide coated products.

Accordingly, one of the objects of the present invention is to provide a method for spraying magnesium oxide onto a metal base which will permit high rates of application resulting in substantially improving the applicationof this type of coating over processes heretofore known. i

Another object of this,invention is to provide a method for covering a 'metal base with a mixture of high melting point refractory oxides one of which is magnesium oxide.

Still another object of this invention is the novel spray magnesium oxide coated articles in which the spray magnesium oxide adheres to the base surface in a novel manner and with a high degree of bond.

3,l8l,954 Patented May 4, 1965 r re.

and the apparatus required consists primarily of a standard oxy-fuel gas torch unit.

In general, the invention method comprises the steps of first maintaining an oxy-fuel gas heating flame, preferably of oxygen and hydrogen in predetermined proportions, introducing the coating material into the combustion zone by suspending it either in the fuel or in the oxygen and passing the particlecarrying mixture to a burner where ignition of the mixture under pressure produces large volumes of flaming combustion gases that are discharged to the outside through a confining passageway that etfects acceleration of the gases to high velocities.

The high temperatures to which the particles can be heated by being entrained in the combusting mixture and in the jet flame whereupon the appreciable temperature increase corresponding to kinetic energy expended upon impact of the high velocity particles upon the surface of the work to be coated make it possible to melt even melting point materials (or at least the lowest melting point constituent of such materials) sutficiently to insure a firm mechanical bond with the surface of the metal body to be coated. It is highly important that the oxy-fuel gas burner be positioned at a predetermined distance from the surface of the base metal at which the tip of the high temperature inner cone of the flame is spaced slightly from that surface while the outer cone of the flame bathes the area immediately surrounding the localized area to be sprayed. The coating material is carried by the combustible mixture and carrier air through the inner cone of the heating flame and impinged onto the heated surface while it is being bathed with the outer flame cone. The magnesium powder contained in the coating material is oxidized by the temperature of the heating flame which approaches but is lower than the metal surface melting temperature. This high velocity imparted to the particles at these temperatures causes them to'deform sufficiently upon impact to weld together to other particles in the coating so as to form a substantially non-porous coating. Moreover, in the method described hereinabove, thethermal and kinetic energy of the coating particles is sutficient so that the surface of the workpiece need not be heated to extremely high temperatures. Consequently, well bonded, non-porous coatings can be produced by the practice of this invention without causing major microstructural change in the workpiece.

A conventional spraying apparatus such as described in Y the United States Patent No. 2,786,779, issued to A. Long et al., or any suitable apparatus known to the art may be used to obtain the coatings of this invention by a thermal oxidation conversion method. However, it is important to use hydrogen and oxygen as the fuel gases because by use of these gases it is possible to obtain a more concentrated spray pattern, a'more complete conversion of the magnesium to magnesium oxide, and an excellent rate of deposition. An optimum flow meter setting of 40 cubic feet per hour for the oxygen and 120 cubic feet per hour for the hydrogen was used for all tested coatings.

The optimum size for the magnesium powder was found to range from 200 to +325 mesh. [Material that stays on a given screen is the oversize or plus that passing through the screen is the undersize or minus This particle size converted to MgO whereas particle sizes ranging from -100 to +200 showed traces of unconverted magnesium. Particle sizes below 325 mesh were precluded because of the possible safety hazard presented.

To improve the properties of the coating, various refracting oxides were added to the magnesium. The additives were selected from the group consisting of cerium oxide, chromium oxide, hafnium oxide, lanthanum oxide and zirconium oxide.

The following examples, illustrative of the articles and method of the invention, are furnished hereinbelow and are to be construed as being merely illustrative and not as limiting the scope of the present invention. By way of exemplifieation, suitable base metals include steel, carbon steel, steel and iron alloys either in the soft or hardened condition, cast iron, nickel and the various nickle alloys such as Monel, Inconel and the like.

Example I A 1010 steel panel was flame sprayed with magnesium powder of 200 mesh. A continental powder gun was used. The oxygen was supplied at a rate of 40 cubic feet per hour and the hydrogen at a'rate of 120 cubic feet per hour. The steel panel was preheated to remove the moisture. The magnesium powder was fed through the hopper on the gun at a rate of 12.5 cubic feet per hour. The spray nozzle was kept a distance of 4 inches from the steel panel. A coating of magnesium oxide was deposited on the steel panel to a thickness of .070 inch of magnesium oxide. This coating was subjected to an erosion test, an adherence and compressibility test and a hygroscopic test.

Example II An Inconel panel was flame sprayed with a mixture of magnesium oxide and cerium oxide. The eerlum oxide was added through the hopper so that the final coating would be 50% CeO and 50% MgO. The ratio of the hydrogen feed to the oxygen feed and the hopper feed were the same as in the previous example. A coating bf CeO and MgO was formed on the Inconel panel te a depth of .065 inch.

Example III Example IV A coating of 50% HfO 50% MgO (by weight) was formed on a steel panel to a depth of .055 inch. T.-.

spraying conditions were the same as in Examples I III. 4

Example V L A coating was sprayed on a steel panel comprising 75% MgO-25% LaO (by weight) to a depth of .065 inch under the conditions hereinabove described.

In the erosion test, the coating were subjected to an (1 air blast from a /z" nozzle positioned 1" from the coating at pressures ranging from 20 to 60 p.s.i. The test duration was 60 seconds at each pressure in the above range.

TABLE I TABLE II Exmnplo Load Coating lnden- Remarks No. (p.s.i thickness tution 1 1. 400 075 010 Severe cracking noted.

""""" 2, 800 075 026 Bond destroyed.

1, 400 005 .010 No cracks. Bond good. 2 2, 800 0135 015 D0.

1 14, 000 .005 020 Cracks in coating. Bond destroyed. 3 1, 400 .005 .005 No (rat-ks. Bond good.

""""" 2. R00 005 .010 Bond destroyed.

1, 400 .055 .003 Excellent. 4 2. S00 055 005 D0.

14,000 .055 .010 Coating chipped around indenture. Bond good.

1, 400 005 .007 Excellent. 5 2,800, 005 010 Do.

14, 000 005 0.21 Bond good.

H ygroscopic test A sample of each coating was placed in a vial of water and allowed to stand 72 hours. All of this coating remained intact.

It will be apparent that the method disclosed in this application teaches a way of obtaining a magnesium oxide coating free of the problems encountered in the past. No attempt is made to claim the method of flame spraying as this was well known in the art. However, this invention teaches a method of producing a flame sprayed magnesium oxide coatingwherein the coating is adherent to many metal and non-metal base materials, wherein it has a melting point of 2800 C. and wherein it is insulative.

The forms of the invention described herein constitute preferred embodiments of the invention and it is to be understood that the invention is not limited to these precise, forms, but may be embodied in other specific formswith-I out departing from the spirit or essential attributes thereof with reference being made to the appended claims rather than to the foregoing description to indicate the scope of the invention. 1

What is claimed is:

1. A method of applying a surface coating, containing 1 at least 50% by weight of magnesium oxide and the balance being a refractory oxide of one from the group consisting of cerium oxide, chromium oxide, hafnium oxide, 3

lanthanum oxide and zirconium oxide, to an article which comprises suspending a mixture of magnesium metal divided to a powder of between 200 and +325 mesh and at least one of the said refractory oxides in a fluid combustible mixture containing oxygen and hydrogen, the oxygen being supplied to the mixture at a rate of 40 cubic feet per hour, the hydrogen being supplied to the mixture at a rate of 120 cubic feet per hour, and the powder being supplied to the mixture at the rate of 12.5 cubic feet per i hour; heating the coating mixture to a high temperature 1 Air pressure (p.s.i.)

Example No. i

I I so Noloss Noloss Small craeks Failed do N 10 1oresopened Pores opened.

No loss No (lo Do. do Do.

ing the steps of:

(1) suspending a mixture of magnesium metal div ided a powder of between -200 and +325 mesh and cerium oxide in a fluid combustible mixture containing oxygen and hydrogen, the oxygen being supplied to the mixture at a rate of 40 cubic feet per hour, the hydrogen being supplied to the mixture at a rate of 120 cubic feet per hour, and the powder being supplied to the mixture at the rate of 12.5 cubic feet per hour;

(2) heating the coating mixture to a high temperature by burning the fluid combustible mixture Whereirt the magnesium metal is oxidized to magnesium oxide;

(3) projecting the heated coating mixture from a spray nozzle disposed 3 to 4 inches from the surface oi: the article to be coated at its flowing temperature whereby an adherent coating of magnesium oxide and cerium oxide is formed upon the article.

3. A method of applying a surface coating of magnesium and chromium oxide to an article, said method comprising the steps of: 5

(1) suspending a mixture of magnesium metal diuided to a powder of between -200 and +325 mesgfiand chromium oxide in a fluid combustible mixture, containing oxygen and hydrogen, the oxygen bein supplied to the mixture at a rate of 40 cubic felt per hour, the hydrogen being supplied to the mixture at a rate of 120 cubic feet per hour, and the powder being supplied to the mixture at the rate of 12.5 cubic feet per hour;

(2) heating the coating mixture to a high temperature by burning the fluid combustible mixture wherein the magnesium metal is oxidized to magnesium oxide;

(3) projecting the heated coating mixture from a spray nozzle disposed 3 to 4 inches from the surface of the article to be coated at its flowing temperature whereby an adherent coating of magnesium oxide and chromium oxide is formed upon the article.

4. A method of applying a surface coating of magnesium and hafnium oxide to an article, said method comprising the steps of:

(1) suspending a mixture of magnesium metal divided to a powder of between -200 and +325 mesh and hafnium oxide in a fluid combustible mixture containing oxygen and hydrogen, the oxygen being supplied to the mixture at a rate of 40 cubic feet per hour, the hydrogen being supplied to the mixture at a rate of 120 cubic feet per hour, and the powder being supplied to the mixture at the rate of 12.5 cubic 1 feet per hour;

i (2) heating the coating mixture to a high temperature by burning the fluid combustible mixture wherein the magnesium metal is oxidized to magnesium oxide; (3) projecting the heated coating mixture from a spray nozzle disposed 3 to 4 inches from the surface of the article to be coated at its flowing temperature whereby an adherent coating of magnesium oxide and hafnium oxide is formed upon the article. 5. A method of applying a surface coating of magnesium an lanthanum oxide to an article, said method comprising the stepsof:

(l) suspending a mixture of magnesium metal divided to a powder of between -20O and +325 mesh and lanthanum oxide in a fluid combustible mixture containing oxygen and hydrogen, the oxygen being supplied to the mixture at a rate of 40 cubic feet per hour, the hydrogen being supplied to the mixture at a rate of cubic feet per hour, and the powder being supplied to the mixture at the rate of 12.5

cubic feet per hour;

(2) heating the coating mixture to a high temperature by burning the fluid combustible mixture wherein the magnesium metal is oxidized to magnesium oxide;

(3) projecting the heated coating mixture from a spray (1) suspending a mixture of magnesium metal divided to a powder of between -200 and +325 mesh and zirconium oxide in a fluid combustible mixture containing oxygen and hydrogen, the oxygen being supplied to the mixture at a rate of 40 cubic feet per hour, the hydrogen being supplied to the mixture at a rate of 120 cubic feet per hour, and the powder being supplied to the mixture at the rate of 12.5 cubic feet per hour; 2) heating the coating mixture to a high temperature by burning the fluid combustible mixture wherein the magnesium metal is oxidized to magnesium oxide; (3) projecting the heated coating mixture from a spray nozzle disposed 3 to 4 inches from the surface of the References Cited by the Examiner UNITED STATES PATENTS 7/60 Haglund 117--22 3/61 White et al. 117-46 FOREIGN PATENTS 402,203 11/33 Great Britain.

RICHARD D. NEVIUS, Primary Examiner. 

1. A METHOD OF APPLYING A SURFACE COATING, CONTAINING AT LEAST 50% BY WEIGHT OF MAGNESIUM OXIDE AND THE BALANCE BEING A REFRACTORY OXIDE OF ONE FROM THE GROUP CONSISTING OF CERIUM OXIDE, CHROMIUM OXIDE, HAFNIUM OXIDE, LANTHANUM OXIDE AND ZIRCONIUM OXIDE, TO AN ARTICLE WHICH COMPRISES SUSPENDING A MIXTURE OF MAGNESIUM METAL DIVIDED TO A POWDER OF BETWEEN -200 AND +325 MESH AND AT LEAST ONE OF THE SAID REFRACTORY OXIDES IN A FLUID COMBUSTIBLE MIXTURE CONTAINING OXYGEN AND HYDROGEN, THE OXYGEN BEING SUPPLIED TO THE MIXTURE AT A RATE OF 40 CUBIC FEET PER HOUR, THE HYDROGEN BEING SUPPLIED TO THE MIXTURE AT A RATE OF 120 CUBIC FEET PER HOUR, AND THE POWDER BEING SUPPLIED TO THE MIXTURE AT THE RATGE OF 12.5 CUBIC FEET PER HOUR; HEATING THE COATING MIXTURE TO A HIGH TEMPERATURE BY BURNING THE FLUID COMBUSTIBLE MIXTURE WHEREIN THE MAGNESIUM METAL IS OXIDIZED TO MAGNESIUM OXIDE, AND THEREAFTER PROJECTING THE HEATED COATING MIXTURE FROM A SPRAY NOZZLE DISPOSED 3 TO 4 INCHES FROM THE SURFACE OF THE ARTICLE TO BE COATED AT AT LEAST ITS FLOWING TEMPERATURE WHEREBY AN ADHERENT COATING OF MAGNESIUM OXIDE AND ONE OF THE SAID REFRACTORY OXIDE IS FORMED UPON THE ARTICLE. 